This is probably a stupid question, but what does "ose" mean? I get that it marks things as sad, but where did it come from? Does it stand for something?

I don’t know exactly where it comes from, but it is short for “morose,” so you can say “ose, ose, and more-ose” when the depressing songs go on just a bit too long.

I saw a list of types of ose songs once, including “scary-ose” and “cheery-ose”, but now I can’t find it (that might have been the whole list, though).